1930年伊東群発地震に伴う地殻変動とダィラタンシー

抄録

A cycle of vertical crustal movement before and after the Ito earthquake swarm, that was active during the period from February to November 1930 in the eastern part of Izu Peninsula, Japan, is thoroughly examined by using all results obtained through precise levelling across its epicentral region in order to investigate a validity of dilatancyfluid diffusion model for earthquake occurrence. Rapidly decaying subsidence followed at the inner part of upheaval area of the ground that was associated with the beginning of the earthquake swarm, while no remarkable subsidence could be recognized at the outer part of the upheaval area for a long duration after the earthquake swarm activity. Some permanent vertical displacement is still remained even for 40 years after the earthquake swarm. Upheaval and subsequent subsidence can be explained by dilatancy due to crack opening and dilatancy recovery due to crack closing. Furthermore, permanent static deformation of the crust supports an elastic dislocation model for the earthquake swarm. The author gives a synthetic model. Dilatancy developed along a future fault plane in the first stage causes local upheaval and slow dislocation movement in the second stage. After the earthquake swarm activity, dilatancy recovery causes subsidence and dislocation gives permanent displacement.